Chapter 11HEAT EXCHANGERS

Heat Transfer

Universitry of Technology Materials Engineering DepartmentMaE216: Heat Transfer and Fluid

ObjectivesRecognize numerous types of heat exchangers, and classify them

Develop an awareness of fouling on surfaces, anddetermine the overall heat transfer coefficient for a heatexchanger

Perform a general energy analysis on heat exchangers

Obtain a relation for the logarithmic mean temperaturedifference for use in the LMTD method, and modify it for different types of heat exchangers using the correction factor

Develop relations for effectiveness, and analyze heatexchangers when outlet temperatures are not known using the effectiveness-NTU method

Know the primary considerations in the selection of

PES OF HEAT EXCHANGERS

pact heat exchanger: It has a large heat fer surface area per unit volume (e.g., car tor, human lung). A heat exchanger with the density > 700 m2/m3 is classified as being act.

s-flow: In compact heat exchangers, the two fluids ly move perpendicular to each other. The cross-s further classified as unmixed and mixed flow.

l-and-tube heat exchanger: The most common type of heat anger in industrial applications.

y contain a large number of tubes (sometimes several hundred) ed in a shell with their axes parallel to that of the shell. Heat

sfer takes place as one fluid flows inside the tubes while the other flows outside the tubes through the shell.l-and-tube heat exchangers are further classified according to theber of shell and tube passes involved.

enerative heat exchanger:lves the alternate passage of theand cold fluid streams through the e flow area.

amic-type regenerator: Involves a ting drum and continuous flow of hot and cold fluid through different ons of the drum so that any on of the drum passes periodically ugh the hot stream, storing heat, then through the cold stream,

cting this stored heat.

denser: One of the fluids is cooled condenses as it flows through the

t exchanger.

er: One of the fluids absorbs heat vaporizes.

e and frame (or just plate) heat exchanger: Consists of a series of plates corrugated flat flow passages. The hot and cold fluids flow in alternate ages, and thus each cold fluid stream is surrounded by two hot fluid streams, ting in very effective heat transfer. Well suited for liquid-to-liquid applications.

-and-frame o-liquid heat

nger.

E OVERALL HEAT TRANSFER COEFFICIENTheat exchanger typically involves two

owing fluids separated by a solid wall. eat is first transferred from the hot fluid to e wall by convection, through the wall by

onduction, and from the wall to the cold uid again by convection. ny radiation effects are usually included in e convection heat transfer coefficients.

U the overall heat transfercoefficient, W/m2C

When

verall heat transfer coefficient U is dominated by the smaller convection cient. When one of the convection coefficients is much smaller than the other hi << ho), we have 1/hi >> 1/ho, and thus U hi. This situation arises frequently

f th fl id i d th th i li id I h fi

overall heat transfer coefficient es from about 10 W/m2C for o-gas heat exchangers to about 0 W/m2C for heat exchangers

nvolve phase changes.

For short fins of high thermal conductivity, we can use this total area in the convectionresistance relation Rconv = 1/hAs

To account for fin efficiency

When the tube is finned on one side to enhance heat transfer, the total heat transfer surface area on

the finned side is

ing Factorerformance of heat exchangers usually deteriorates with time as a result of

mulation of deposits on heat transfer surfaces. The layer of deposits represents onal resistance to heat transfer. This is represented by a fouling factor Rf.

uling factor increases with the operating temperature and the length of e and decreases with the velocity of the fluids.

ALYSIS OF HEAT EXCHANGERSngineer often finds himself or herself in a positionselect a heat exchanger that will achieve a specified temperature

hange in a fluid stream of known mass flow rate - the log mean emperature difference (or LMTD) method.

predict the outlet temperatures of the hot and cold fluid streams in specified heat exchanger - the effectiveness–NTU method.te of heat transfer in heat nger (HE is insulated): Two fluid

streams that have the samecapacity rates

experience the same

temperature change in a well-

insulated heat exchanger.

Variation of fluid temperatures in a heat exchanger when one of the fluidscondenses or boils.

he rate of evaporation or condensation of the fluidhe enthalpy of vaporization of the fluid at the specified temperature or pressure.at capacity rate of a fluid during a phase-change process must approach since the temperature change is practically zero.

T i ( )

E LOG MEAN TEMPERATURE DIFFERENCE THOD

Variation of the fluid temperatures in aparallel-flow double-pipe heat exchanger.

log mean temperature

The arithmetic mean temperature difference

The logarithmic mean temperaturedifference Tlm is an exact representation of the average temperature difference between the hot and cold fluids. Note that Tlm is always less than Tam. Therefore, using Tam in calculationsinstead of Tlm will overestimate the rate of heat transfer in a heat exchanger between the two fluids. When T1 differs from T2 by no more than 40 percent, the error in using the arithmetic mean temperature difference is less than 1 percent. But the error increases toundesirable levels when T1 differs from T2 by greater amounts.

Counter-Flow Heat ExchangersIn the limiting case, the cold fluid will be heated to the inlet temperature of the hotfluid. However, the outlet temperature of the cold fluid can never exceed the inlet temperature of the hot fluid.For specified inlet and outlet temperatures, Tlm a counter-flow heat exchanger is always greater than that for a parallel-flow heat exchanger. That is, Tlm, CF > Tlm, PF, and thus a smaller surface area (and thus a smaller heat exchanger) is needed to achieve aspecified heat transfer rate in a counter-flow heat exchanger.

When the heat capacity rates of the two fluids are equal

Multipass and Cross-Flow Heat Exchangers:Use of a Correction Factor

F correction factor depends on the geometry of the heat exchanger and the inlet and outlet temperatures of the hot and cold fluid streams.F for common cross-flow and shell-and-tube heat exchanger configurations is given in the figure versus two temperature ratios P and R defined as

1 and 2 inlet and outletT and t shell- and tube-side temperatures

F = 1 for a condenser or boiler

Correction factor F charts for common shell-and-tube heat exchangers.

Correction factor F chartsfor common cross-flow heat exchangers.

e LMTD method is very suitable for determining the size of a heat exchanger to realize prescribed outlet temperatures when the mass flow rates and the inlet and outlet temperatures of the hot and cold fluids are specified.

th the LMTD method, the task is to select a heat exchanger that will meet the prescribed heat transfer requirements. The procedure to be followed by the selection process is:

Select the type of heat exchanger suitable for the application.

Determine any unknown inlet or outlet temperature and the heat transfer rate using an energy balance.

Calculate the log mean temperature difference Tlm and the correction factor F, if necessary.

Obtain (select or calculate) the value of the overall heat transfercoefficient U.Calculate the heat transfer surface area As .e task is completed by selecting a heat exchanger that has a

HE EFFECTIVENESS–NTU METHODond kind of problem encountered in heat exchanger analysis is themination of the heat transfer rate and the outlet temperatures of the hot and uids for prescribed fluid mass flow rates and inlet temperatures when the nd size of the heat exchanger are specified.

transfer effectiveness

aximum possible heat transfer rate

Actual heat transfer rate

effectiveness of a t exchanger depends he geometry of the t exchanger as well he flow arrangement. refore, different types eat exchangers have rent effectiveness tions.

llustrate the elopment of the ctiveness e relation he double-pipe allel-flow heat hanger.

ctiveness relations of the heat exchangers typically involve theensionless group UAs /Cmin.

quantity is called the number of transfer units NTU.

For specified values of U and Cmin, the value of NTU is a measure of the surface area As. Thus, the larger the NTU, the larger the heat exchanger.

capacity ratio

effectiveness of a heat exchanger is a function of the ber of transfer units NTU and the capacity ratio c.

Effectiveness for heat exchangers.

n all the inlet and outlet temperatures are specified, the size of heat exchanger can easily be determined using the LMTD

(e.g., boiler, condenser)

bservations from the effectiveness relations and chartsThe value of the effectiveness ranges from 0 to 1. It increases rapidly with NTU for small values (up to about NTU = 1.5) but rather slowly for larger values. Therefore, the use of a heat exchanger with a large NTU (usually larger than 3) and thus a large size cannot be justified economically, since a large increase in NTU in this case corresponds to a small increase in effectiveness.For a given NTU and capacity ratio c = Cmin /Cmax, the counter-flow heat exchanger has the highest effectiveness, followed closely by the cross-flow heat exchangers with both fluids unmixed. The lowest effectiveness values are encountered in parallel-flow heat exchangers.The effectiveness of a heat exchanger is independent of the capacity ratio c for NTU values of less than about 0.3.The value of the capacity ratio c ranges between 0 and 1. For a given NTU, the effectiveness becomes a maximum for c = 0(e.g., boiler, condenser) and a minimum for c = 1 (when the heat

LECTION OF HEAT EXCHANGERSuncertainty in the predicted value of U can exceed 30 percent. Thus, it is atural to tend to overdesign the heat exchangers.transfer enhancement in heat exchangers is usually accompanied by

ncreased pressure drop, and thus higher pumping power. efore, any gain from the enhancement in heat transfer should be weighed gainst the cost of the accompanying pressure drop. ally, the more viscous fluid is more suitable for the shell side (larger assage area and thus lower pressure drop) and the fluid with the higher

pressure for the tube side.proper selection of at exchanger depends everal factors:eat Transfer Rate

Costumping Powerize and Weightype

The annual cost of electricity associated withthe operation of the pumps and fans

The rate of heat transfer in the prospective heat exchanger

SummaryTypes of Heat ExchangersThe Overall Heat Transfer Coefficient Fouling factor

Analysis of Heat ExchangersThe Log Mean Temperature DifferenceMethod Counter-Flow Heat Exchangers Multipass and Cross-Flow Heat Exchangers:

Use of a Correction Factor

The Effectiveness–NTU MethodSelection of Heat Exchangers